There has been past threads in the V10 forums of guys using heavy 15w-40 and 20w50 in the 6.8l V10s. In the case of the old school auto mechanic that was running his V10 with 20w50 his engine lasted something like 15k or so before it melted down.

There is absolutly no reason to use anything other than what is recommended in the 6.8l, 5w-20 earlier V10s prior to 2000 called for 5w-30. 5w-30 will work good in the V10. Using a heavier oil in the V10 or other mod motor is down right moronic, even if just using 10w-30. 5w-20 is proven to run good in modulars so why not run it?

Mod motors need the lighter oil. The camshafts in Modular motors actually ride on the machined aluminum head surface. There are no bearings that it rides on, just steel to aluminum. Oil needs to get between the cam journals and the head for proper lubrication and thick oils just won't do that.